Simple core body thermometers without heating (hereafter referred to as non-heating core body thermometers) are used to measure the temperature of a core part of a body.
Non-heating core body thermometers known in the art may either include one heat flux sensor (temperature difference sensor) with temperature sensors arranged separately on the upper and lower surfaces of a heat insulator having a relatively large area to measure the core body temperature, or include two such heat flux sensors to measure the core body temperature.
A non-heating core body thermometer that includes one heat flux sensor will now be described with reference to FIGS. 25A to 25C.
To measure the core body temperature using a non-heating core body thermometer including one heat flux sensor, the heat flux sensor is placed in tight contact with a body surface as shown in FIG. 25A.
As shown in FIG. 25B, Tt, which is the temperature of the lower surface of the heat insulator measured with a heat flux sensor placed in tight contact with the body surface (a temperature sensor on the lower surface of the heat insulator), is lower than the core body temperature Tb, and Ta, which is the temperature of the upper surface of the heat insulator measured with a heat flux sensor placed in tight contact with the body surface (a temperature sensor on the upper surface of the heat insulator), is lower than the temperature Tt. The heat equivalent circuit for the structure shown in FIG. 25A can be expressed as shown in FIG. 25C. In the figure, Rx is the thermal resistance of subcutaneous tissue, which is a non-heating part, and R1 is the thermal resistance of the heat insulator.
When the temperature of each part of the heat flux sensor placed in tight contact with the body surface is stabilized, the quantity of heat passing through the non-heating part per unit time is equal to the quantity of heat passing through the heat insulator per unit time. More specifically, when the temperature of each part of the heat flux sensor is stabilized, Formula 1 below holds.(Tb−Tt)/Rx=(Tt−Ta)/R1  (1)
Thus, when the temperature of each part of the heat flux sensor is stable, the core body temperature Tb can be calculated using Formula 2 below, which is obtained by solving Formula 1 for Tb.Tb=Tt+(Tt−Ta)·Rx/R1  (2)
The non-heating core body thermometer that measures the core body temperature using the single heat flux sensor calculates the core body temperature Tb using Formula 2.
The value of Rx varies depending on positions as well as individual persons. When the core body temperature Tb is calculated using a fixed value as the Rx value in Formula 2, the measurement result will include a measurement error in accordance with a difference between the Rx value used in Formula 2 and an actual value of Rx.
The non-heating core body thermometer that measures the core body temperature using two heat flux sensors has been developed to reduce such measurement errors in the measurement result of the core body temperature Tb.
To measure the core body temperature using this type of non-heating core body thermometer, as shown in FIG. 26A, the two heat flux sensors are placed in tight contact with the body surface.
As shown in FIGS. 26A and 26B, Ta is the temperature of the upper surface of the heat insulator measured using the thermal resistance of the heat insulator of one heat flux sensor (first heat flux sensor), and Tt is the temperature of the lower surface of the heat insulator measured using the thermal resistance of the heat insulator of the first heat flux sensor, whereas Ta′ is the temperature of the upper surface of the heat insulator measured using the thermal resistance of the heat insulator of the other heat flux sensor (second heat flux sensor), and Tt′ is the temperature of the lower surface of the heat insulator measured using the thermal resistance of the heat insulator of the second heat flux sensor. The heat equivalent circuit for the structure shown in FIG. 26A can be expressed as shown in FIG. 26C. In the figure, Rx is the thermal resistance of subcutaneous tissue, which is a non-heating part, R1 is the thermal resistance of the heat insulator in the first heat insulator, and R2 is the thermal resistance of the heat insulator in the second heat insulator.
As a result, Formula 2 described above holds for the first heat flux sensor, whereas Formula 3 below holds for the second heat flux sensor.Tb=Tt′+(Tt′−Ta′)·Rx/R2  (3)
Eliminating Rx from Formulas 2 and 3 yields Formula 4 below.
                    Formula        ⁢                                  ⁢        1                                                            Tb        =                                            R              ⁢                                                          ⁢              2              ⁢                              (                                  Tt                  -                  Ta                                )                            ⁢                              Tt                ′                                      -                          R              ⁢                                                          ⁢              1              ⁢                              (                                                      Tt                    ′                                    -                                      Ta                    ′                                                  )                            ⁢              Tt                                                          R              ⁢                                                          ⁢              2              ⁢                              (                                  Tt                  -                  Ta                                )                                      -                          R              ⁢                                                          ⁢              1              ⁢                              (                                                      Tt                    ′                                    -                                      Ta                    ′                                                  )                                                                        (        4        )            
Formula 4 can be transformed into Formula 5 below using the ratio k of R2 to R1 (=R2/R1).
                              Formula          ⁢                                          ⁢          2                ⁢                                                                                      Tb        =                                                            k                ⁡                                  (                                      Tt                    +                    Ta                                    )                                            ⁢                              Tt                ′                                      -                                          (                                                      Tt                    ′                                    -                                      Ta                    ′                                                  )                            ⁢              Tt                                                          k              ⁡                              (                                  Tt                  -                  Ta                                )                                      -                          (                                                Tt                  ′                                -                                  Ta                  ′                                            )                                                          (        5        )            
The non-heating core body thermometer that measures the core body temperature using the two heat flux sensors calculates the core body temperature Tb using Formula 4 or 5 above.